Luciferin is a racemic compound that, in solution, racemizes from D-luciferin to L-luciferin or from L-luciferin to D-luciferin. D-luciferin can be utilized as a substrate by the enzyme luciferase to generate light, whereas L-luciferin is largely inhibitory and causes a decrease in luminescence. Slow racemization of D-luciferin to L-luciferin during storage presents problems for assays that exploit the activity of luciferase due to the varying and decreased availability of D-luciferin substrate, and the inhibitory effects of L-luciferin.